1. Field of the Invention
This invention resides in the chemical arts. In one aspect, it relates to synthetic resins, particularly poly(alkylene carbonates). In another aspect, it relates to photolithography.
2. Description of Related Art
Poly(alkylene carbonates) are polymers in which the basic monomeric unit is represented by the generic skeletal formula: ##STR1##
Depending on the molecular weight of the polymers, their state ranges from normally viscous liquid to normally solid. The expressions "nomrally liquid" and "normally solid" as used in this specification mean liquid and solid at 20-25.degree. C. One process for making them comprises the catalytic copolymerization of carbon dioxide and epoxides such as, for example, ethylene oxide, propylene oxide, cyclohexene oxide, and the like. In the commercial practice of this process, the product is a normally solid polymer in which the polymer chains are composed predominately of alkylene carbonate units. However, the chains can have as much as 20 mole % alkylene ether units. These units are represented by the generic skeletal formula: ##STR2##
However, it is preferred that the chains have 8 mole % or less of such units. Also, in such process the desired, normally solid polymer product usually is separated from the polymerization reaction mixture, and washed with aqueous media to remove catalyst and by-products.
One utility of these commercial products in general is as thermally decomposable, sacrificial binders in the manufacture of metal and ceramic articles. In this regard, at temperatures above 300.degree. C., but substantially less than 550.degree. C., poly(alkylene carbonates) in general have the property of decomposing and burning cleanly and completely with minimal ash residue.
The U.S. Pat. No. 3,900,424 to Inoue et al. discloses the copolymerization of epoxy compounds with carbon dioxide to form normally solid polymers in which alkylene oxide units and carbon dioxide units alternate. Included in the patient's listing of exemplary epoxy compounds are ethylene oxide, propylene oxide, cyclohexene oxide, glycidyl methacrylate, vinylcyclohexene oxide, and mixtures of the listed compounds. The patent discloses separating the polymer product from the polymerization reaction mixture with dilute hydrochloric acid and then water washing the separated product. No utility of the polymers formed by the copolymerization is specifically alleged in the patent.
The U.S. Pat. No. 3,248,415 to Stevens discloses polymers having terminal hydroxyl groups and molecular weights of 700 to 5000, which are made by copolymerizing with a base catalyst a mixture of carbon dioxide, a 1,2 epoxide and a polyhydric alcohol. The structural formula for a typical polymer made with ethylene oxide as the 1,2 epoxide is disclosed as: ##STR3##
wherein m, n, o, etc., are positive whole integers of 1 or more, which can be the same or different. Similar polymers having average molecular weights between about 1,5000 and 30,000 are disclosed by the U.S. Pat. No. 4,500,704 to Kruper et al.
The U.S. Pat. Nos. 4,066,630, 4,104,264, 4,145,525 and 4,303,759 to Dixon et al., disclose normally solid poly(alkylene carbonates) apparently made by the process of the above Inoue et al. patent. These polymers are described as being of high molecular weight, such being about 50,000 to 500,000, with the polymer chains being hydroxy terminated. However, under the concepts of the invention of this specification, the number of polymer chains and thus the number of hydroxy groups available for reaction with compounds having substituents suitable for cross-linking are too few for the amount of cross-linking of polymer chains that needs to take place in photolithographic application.
As to the photolithographic aspect of the invention, a photoresist is a composition which is used by applying it to a substrate to form a thin (for example 0.5-1 .mu.m thick) layer thereon, and subsequently exposing the latter in an image-wise fashion through a mask to molecular energizing radiation. Examples of such radiation include light, particularly ultraviolet light, and electrons, x-rays, ions and the like. The mask contains areas that block the passage of radiation through the mask, and areas through which radiation can pass. These areas define a pattern, for example, an electronic circuit pattern. The photoresist composition contains radiation sensitive material that chemically responds to the incident radiation reaching the resist layer to form a latent image of the mask pattern. The latent image is then "developed" by solvent or other treatment, depending on the kind of photoresist composition that is involved. The resist layer remaining on the substrate after development protects the underlying substrate during the additive, subtractive, or both, treatments of the areas of the substrate from which the resist layer was removed. If, for example, the underlying substrate is silicon dioxide, immersion of the structure in an etchant such as buffered hydrofluoric acid will result in selective etching of the substrate in those areas made bare by the development step.
One kind of photoresist composition comprises cross-linkable polymer material that is soluble in a development solvent, but, when cross-linked, becomes insoluble in that solvent. The polymer material contains functional groups that can be made to react to form cross-linked networks of polymer chains. The composition also comprises initiator material which, when exposed to the above described radiation, causes the functional groups to react as indicated. This invention relates to this kind of a photoresist composition.
A problem to which this invention provides a solution has to do with the removal of what remains of the photoresist layer from a substrate after the polymer material has been cross-linked, and the resist layer has served its purpose. One method to remove the layer is to thermally decompose it. However, most polymers heretofore in photoresists, when in cross-linked condition, have one or more or these disadvantages: they do not readily thermally decompose; they leave objectionable solid residues when they have decomposed; they emit toxic or odorous, or both, fumes as they thermally decompose. One or more of these disadvantages exist in the case of poly(methyl methacrylate), a polymer widely used in photoresist compositions. There is a need, therefore, for a photoresist composition in which the polymer material, when cross-linked, is readily thermally decomposed, leaves an insignificant, if any, amount of solid residue, and emits unobjectionable fumes as it thermally degrades.